The frames of heavy duty trucks are typically suspended on torque reactive rear suspensions. Commonly, these suspensions provide a leaf or air spring to maintain a constant vehicle height. As drive train systems are optimized to provide greater torque output, suspensions in general have been made stiffer to compensate for torque reaction and driveline vibrations that are associated with the higher torques. While such compensation has diminished the effects of torque reaction and driveline vibrations to provide a relatively comfortable ride during normal operating conditions, the stiffer suspensions cannot absorb high impact forces such as those caused by rough roads and as a result do not provide adequate cushioning against such events.
Several techniques have been used in the art to adaptively control the stiffness characteristics of vehicle suspension components, such as shock absorbers or air springs, in response to operating characteristics. For example, U.S. Pat. No. 4,564,214 to Tokunaga et al. discloses a shock absorber having an air chamber that serves as an integral air spring. The air chamber is in fluid communication with an auxiliary reservoir to provide a relatively “soft” ride during normal operating conditions. When the steering wheel is turned, the air chamber is disconnected from the auxiliary reservoir, which results in a smaller reservoir and stiffer ride characteristics, to provide a relatively “hard” ride during the steering event to enhance vehicle control. U.S. Pat. No. 6,276,710 to Sutton discloses a system of air springs for a vehicle tandem axle in which air springs on the same side of the vehicle are selectively placed in fluid communication with one another to provide a relatively “soft” ride by virtue of effectively increasing the volume of air in the reservoir over the volume of air in the reservoir of each air spring by itself. When the vehicle body begins to roll, the air springs are disconnected to provide better control until the body returns to its normal orientation.